Tractate Parah - Chapter 7 - Mishnah 9 פרה פרק ז׳ משנה ט׳

One was carrying his water on his shoulder and he ruled in a matter of law, or showed others the way, or killed a snake or a scorpion, or picked up food in order to store it, it [the water] is invalid; But [if he picked up] food to eat, it is valid. [And if he killed] a snake or a scorpion that hindered him, it remains valid. Rabbi Judah said: this is the general rule: If the act was is in the nature of work, the mixture is invalid whether the man stopped or not, If it was not in the nature of work: If he stopped, it is invalid; But if he did not stop it remains valid.